Vehicle brakes are known whereby a brake rotor, such as a brake disc or a brake drum is attached to a vehicle wheel so as to rotate with that vehicle wheel. Non-rotating friction material, such as friction material on a brake pad or on a brake shoe can be frictionally engaged with the rotor so as to slow or stop the vehicle.
When it is not required to slow or stop the vehicle, the brake system is arranged so that a relatively small clearance, known as a running clearance, is provided between the rotor and the friction material. This running clearance is arranged to prevent brake drag. Brake drag is an unnecessary frictional drag, which causes increased fuel consumption, and increased wear of brake components. Accordingly, brake drag is undesirable.
WO99/06725 shows an example of a mechanically operated brake wherein the running clearance is automatically adjusted as the brake pads wear as a result of normal engagement and disengagement of the brake. WO99/06725 operates such that in the event there is excessive running clearance an initial movement of the brake pads towards the brake disc is caused solely by operation of an operating lever. Once the operating lever has passed through a predetermined distance (and the brake pads have not yet engaged the brake disc) then subsequent movement of the brake pads towards the brake disc is caused by a combination of the operating lever and the adjuster mechanism being adjusted to reduce the running clearance. Once the brake pad has engaged the brake disc the adjuster mechanism is unable to continue adjusting. As will be appreciated, adjustment of the brakes of WO99/06725 takes place as the brakes are being applied. This adjustment is in the direction which reduces the running clearance. WO99/06725 is not capable of automatically adjusting the brakes to increase the running clearance.
The automatic adjustment mechanism is designed to provide a predetermined running clearance for the brake. The running clearance is typically determined during the design process of the brake and depends upon various factors including rotor thickness, friction material thickness and other criteria of the brake assembly and associated vehicle.
One consideration when designing the running clearance is that the vehicle must be able to brake from its maximum speed or on a steep descent to a standstill and then, immediately be able to release the brakes and drive on. As will be appreciated, when a vehicle brakes from its maximum speed or on a steep descent to a standstill a considerable amount of kinetic energy is converted into heat energy by the brakes. This heat energy causes various brake components to heat up and therefore expand. In particular, where the brake is a disc brake, the brake disc and the friction material of the brake pads will expand. This expansion may cause the running clearance of a hot brake to be smaller than the running clearance of a cool brake. Accordingly, the normal cool running clearance of a brake must be sufficiently large such that when the brake becomes hot (for example after being braked to a standstill from a maximum running speed of the associated vehicle) there still exists a running clearance to enable the vehicle to continue its journey. If the cool running clearance is too small, when the vehicle has braked to a standstill from its maximum speed, the friction material may expand into engagement with the rotor, and the vehicle will be unable to continue its journey, until such time as the brake components have cooled down, or there may be drag which will be undesirable.
However, when brakes which have a running clearance are applied, the friction material has to be moved over the distance of the running clearance before any braking force is applied to the rotor. Thus there is a short time delay between when the brake pads start to move, and when they start to apply a braking force.
Accordingly, in known brake designs the running clearance must be sufficient to ensure a running clearance when the brake is both hot and cool and to ensure that brake drag does not occur, but must not be so large so as to cause a significant time delay between when the brake pads start to move and when a brake force is applied to the rotor.
With pneumatically operated mechanically adjusted brakes a particular problem can occur when the vehicle is parked with hot brakes. Certain types of friction material are compressible and the compressibility increases with temperature. Therefore, when the vehicle is parked with hot brakes, i.e. the parking brake is applied whilst the friction material and the rotor is hot, the friction material is compressed to a thickness less than it would have been had the vehicle been parked whilst the friction material and rotor were cold and this is in spite of the fact that the friction material, in an uncompressed form, typically expands as the temperature increases. As the friction material and rotor cools, for example over night when the vehicle is not being used, the friction material remains in this compressed state. When the park brake is released at a cool ambient temperature, the friction material does not immediately return back to its original thickness, rather it remains “set” in a compressed state, and this is in spite of the fact that since the park brake is no longer applied, there is no compression force on the friction material. In one example, the change in running clearance may be 0.7 mm (i.e. 0.35 mm per brake disc pad). Furthermore, the rotor will have cooled and where the rotor is a disc, the disc will have become thinner due to the thermal contraction of the disc material as it cools.
The net result of the cool rotor and the cool friction material being in a “set” compressed condition is that when the park brake is released the running clearance will be increased. The adjuster mechanism may detect this increase in running clearance and therefore adjust so as to reduce the running clearance to the desired running clearance.
However, as the brake is used the friction material goes through a thermal cycle upon each application of the brake and this “conditioning” of the friction material causes the “set” to be released thereby causing the friction material to expand (or swell). Note this expansion is in addition to any expansion caused by the increase in temperature of the friction material. “Conditioning” typically requires several (a plurality) of applications and releases of the brake. However, the adjustment of the brake typically occurs during the first application of the brake, in particular as the running clearance is being closed for the first time after release of the park brake. In other words adjustment of the brake occurs prior to “conditioning” of the friction material. Because of the order in which the adjustment and the “conditioning” takes place, this can result in a brake having a correct running clearance when the friction material is ‘set’ in a compressed condition, but after conditioning this running clearance becomes too small, or potentially non-existent as the friction material expands during the conditioning process. Clearly, if the actual running clearance reduces to zero, then brake drag occurs causing heat which in turn causes expansion of the friction material and the rotor which causes further brake drag, further heat, further expansion etc. Clearly such a situation is undesirable. Note that in this situation, whilst the operator may not be applying the brake in the sense of the operator's foot will not be on the brake pedal or the like, nevertheless the brake is applied in the sense of the friction material is in rubbing engagement with the rotor causing significant drag on the rotor.
In particular, this situation applies to mechanically (e.g. pneumatically) operated disc brakes having mechanical adjusters since the mechanical adjusters are only capable of operating to reduce the running clearance and are incapable of increasing the running clearance.
Conditions in which friction material takes a compression set may cause alternative issues with mechanically (e.g. pneumatically) operated brakes wherein adjustment is carried out by electrical means, such as electric motors. In order to save weight and cost, the electric motors are sized such that they are only capable of adjusting a running clearance when the brake is not applied. When the brake is applied, since the force of application of the brake passes through the adjuster mechanism, then the internal friction of the adjuster mechanism increases significantly, to such an extent that electric motor cannot adjust the brake.
EP2650556 shows an example of an electrically actuated adjuster mechanism which endeavors to maintain a constant running clearance. EP2650556 is a mechanically operated brake, i.e. a mechanical actuator, in this case an air actuator operates to apply the brake. EP2650556 is not an electrically actuated brake, i.e. it does not include an electric motor which applies braking force. The adjuster mechanism of EP2650556 is a distinct component from the actuator of EP2650556. The actuator performs the function of applying and releasing the brake. The adjuster mechanism performs the function of adjusting (in particular the running clearance) of the brake. The adjuster mechanism is not used to apply the brake. The actuator mechanism is not used to adjust the running clearance of the brake. The actuator is an air operated actuator but other types of mechanical actuator may be used. The adjuster mechanism is electrically operated, i.e. in order to adjust the adjuster mechanism the electric motor must be operated.
In one embodiment the system of EP2650556 determines an amount of running clearance as the brake is applied. Upon subsequent release of the brake an appropriate adjustment to the running clearance is made. Thus, in this particular embodiment, no adjustment of the brake is made during application of the brake, rather it is made after the brake has been released. If the brake is released after the friction material has taken a compressed set, the instantaneous running clearance will be larger than the expected running clearance by an amount equal to the amount of compression set of the friction material. However, the brakes will need to be re-applied, in order to determine the instantaneous running clearance, and then upon further release of the brakes is the adjustment then made.
For the avoidance of doubt, the amount of wear of friction material during conditioning is negligible, and can be ignored for the present purposes. In one example, the majority of conditioning takes place between the first and tenth applications of the brake following the vehicle being parked overnight, e.g. in the first few minutes of operation of the vehicle.